Method for producing reflective taps in optical fibers and applications thereof

ABSTRACT

A method for producing reflectors in a continuous length of optical fiber is disclosed. The present process includes the steps of preparing the ends of two or more optical fibers, placing one or more of these fibers in a vacuum system and applying a metallic or dielectric coating to the fiber ends, and then fusing the prepared, coated ends of the fibers together until the reflectivity of the region reaches a desired value.

This is a divisional, of application ser. no. 108,270 filed Oct. 13,1987, now U.S. Pat. No. 4,848,999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method for producingreflectors in continuous lengths of optical fiber. More particularly,the present invention involves the fabrication of reflectors in opticalfibers by fusion splicing. The present invention also relates toapparatus which make use of fiber reflectors produced as a result of theclaimed invention.

2. Description of the Prior Art

A number of methods for fabricating taps in optical fibers have beendisclosed in the art. One such method involves the bending of theoptical fiber axis in order to couple out some of the light which wouldordinarily propagate through the substantially linear fiber. This methodis used, for example, in the Siecor Model M67-210Local InjectionDetection System, as a means of monitoring the transmitted power in afusion splicing unit. The tap produced as a result of this method ofremoving light is normally referred to as a macrobend tap.

A second method known in the art discloses the use of two fibers havingrelatively thin cores which are merged or placed in close proximity toeach other so that at least some of the propagated light couples fromone fiber to another. Such a method is generally disclosed in suchcommercial applications as the Amphenol Model 945 Fiber Coupler, oftenreferred to as a fiber optic directional coupler.

Yet a third prior art method discloses the use of a reflective filmdeposited or evaporated on the ends of oplique optical fibers which arethen physically combined at a joint or other similar bonding structure.In such prior art applications, a dielectric film, such as TiO₂ or SiO₂,is alternatively evaporated on the fiber ends in order to reflect partof the light propagated through the fiber. Couplers of this typegenerally utilize optical fibers cut at an angle of 45° to theirrespective axis, and utilize optical cement for joining the fiber ends.

These prior art methods, however, suffer from a number of disadvantages.When light is coupled out by bending or distorting the linear axis ofthe optical fiber, it oftentimes becomes difficult to concentrate thepropagated light into a small area photodetector. This disadvantagehinders high speed operation desirous in contemporary signal processingapplications. Further, such a method couples light out in only a forwardor lateral direction, but not in the reverse direction, as is oftenrequired in preferred applications.

Another significant disadvantage of prior art methods based uponmacrobend or directional coupler taps is their tendency to introduce"modal noise" into the coupled fiber system. Recognizable as a spuriousamplitude modulation at the receiver, modal noise is caused by thehighly mode selective nature of the optical taps normally createdbetween spliced fiber ends.

Other disadvantages of prior art methods include the general lack ofstructural integrity associated with the cementing of optical fiberscoated with dielectric films. For practical applications for this kindof coupler, therefore, mechanical support is generally needed, whichsupport greatly increases the overall bulk and expense of the finalsystem.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned and otherdisadvantages by providing an inexpensive method for producingreflectors in continuous lengths of optical fiber.

In a preferred embodiment of the present invention, optical fibers arefirst prepared by cleaving or polishing their ends so as to establish aflat, smooth bonding surface. After taking necessary precautions toremove organic films or other contaminants, a selected portion of thesefibers are placed in a vacuum system and the prepared ends are thencoated with a metallic material such as Ti or a dielectric material suchas TiO₂. This coating can be prepared by using thermal evaporation,electron beam evaporation, or sputtering. After this coating has beenapplied to the selected fibers, the prepared, coated fibers are removedfrom the vacuum system and prepared for splicing. A reflector is thencreated in a prepared, coated fiber by placing one of these coatedfibers end-to-end with an uncoated prepared fiber in a fusing splicingapparatus. One or more arc discharges are then produced by activatingthe fusion splicing apparatus until the fiber ends are joined togetherand the reflectivity of the spliced region reaches a desired value.

Using a general aspect of the above described method, variablyreflective optical taps may be produced by utilizing the dielectric tometal phase transitions of vanadium oxides which have been incorporatedin reflectors formed in a length of optical fiber. By passing electriccurrent through a resistive wire or film formed along a length of fibercontaining the reflector, the reflector is heated sufficiently to drivethe selected vanadium oxide coating through this phase transition, thusaltering the overall reflectivity of the fiber mirror. When the currentis turned off or reduced, the optical reflector fiber cools sufficientlyto bring it back through the phase transition to its originalreflectivity.

Variably reflective optical taps may also be formed utilizingFabry-Perot cavities arranged along a length of optical fiber. TheFabry-Perot cavities themselves are formed using much of the methodologydescribed earlier for the formation of optical taps. An electricallyresistive medium is then deposited along the fiber segment between theoptical reflectors such that the introduction of an electric currentthrough the medium brings about a calculated change in reflectivity.

Optical fiber taps produced as a result of the aforedescribed methodhave a variety of applications. One of the primary applications forthese fiber reflectors are as taps in tapped delay line filters, ortransversal filters. The use of variable reflectors in such taps alsoserves to make the tap weight programmable. Also, and similar to othertypes of transversal filters, these optical taps can function as matchedfilters, correlators, waveform or sequence generators, and deconvolvers.

Using the aforedescribed methods, optical fibers can be prepared withthe surface normal to the axis or with the surface at an angle. When theangle between the surface and the axis is 45°, for example, the lightwill be reflected out of the film in a direction approximately normal toits axis. A fiber with this sort of surface to axis configuration couldbe used as a tapping element in fiber communication systems or in afiber sensor, thus providing an alternative to a directional coupler.

A variable reflectivity optical tap incorporating a vanadium oxide mayhave significant utility in applications where it is necessary todetermine and quantify a fluctuation in system temperature. One suchexample is a thermal sensor such as may be utilized in a progressivelymonitorable fire alarm system. In optical taps incorporating vanadiumoxides, a dramatic change in reflectance will occur near thesemiconductor-to-metal phase transition temperature. With such opticaltap systems placed at desired locations along a continuous fiber line ina building or naval vessel, for example, the progression of a fire maybe monitored via a pulsed light source. Since silica fibers canwithstand very high temperatures, this type of system should continue tofunction properly even when a part of the fiber cable is engulfed inflames.

Fabry-Perot interferometers consisting of single mode fibers withoptical mirrors disposed at their ends have been used in the art asinterferometric sensors and as discriminators for coherent communicationsystems. Using the method of the present invention, Fabry-Perotinterferometers may be incorporated into a continuous length of opticalfiber. In this fashion also, low-finesse Fabry-Perots, monitored inreflection, may be used as temperature sensors.

The taps produced as a result of the present invention are stronglypolarization-selective. This polarization selectivity may be utilized insingle mode systems, such as in fiber gyroscopes, which necessitatepolarizing elements. In such applications, the mirrors produced in thetaps would be oriented at Brewsters angle so that all of the lightpolarized in a first plane is transmitted through the mirror, whilelight polarized in a second plane is partially reflected. Thus, a highlypolarized, transmitted beam may be obtained using multilayer quarterwave coatings.

It is appreciated in the art that reflection from external cavities canlead to mode stabilization and line narrowing in diode lasers. Themirrors produced as a result of the present invention, in conjunctionwith pigtailed lasers, are ideal for this application. For modelockingapplications, an array of mirrors may be utilized. In such a case, theround trip delay phase between the mirrors could equal the spacing ofthe mode locked pulses and would approximate the inverse of the laserrelaxation frequency.

The present invention has many advantages over the prior art. One chiefadvantage is the ability to produce reflectors simply and inexpensivelyin continuous lengths of optical fiber. In signal processingapplications, this would allow the fabrication of transversal filtersfor operation with very high bandwidth analog (e.g. > lGHz)) or highdata rate (e.g. > lG bit/sec) digital signals.

Another advantage of the present invention is the overall mechanicalstrength of the resultant tap. Using the method of the claimedinvention, the strength of the fiber material may be maintained at thetap site so that no mechanical support is needed, thus enhancing theflexibility of the junction while decreasing the overall bulk of thesystem.

Another advantage of the present invention is the ability to fabricatean optical tap that can be made to reflect light out of the side of afiber, or to reflect it back down the fiber axis.

Yet another advantage of the claimed method is that the reflectance ofthe resultant tap can be adjusted to a desired value at the time the tapis being produced.

Yet another advantage of the present invention is the simple, efficientremoval of light from a multi-mode fiber so as to substantially reducemodal noise. The method of the present invention results in an opticaltap having uniform reflectance across the fiber cross-section. With suchuniform reflectance, modal noise produced by the coupler itself is muchlower than for directional coupler taps in multi-mode systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. la-e generally illustrate the basic process of the presentinvention by which a reflector may be formed in a continuous length ofoptical fiber.

FIG. 2 schematically illustrates an optical time domain reflectometer(OTDR). The configuration illustrated in FIG. 2 also finds applicationas a tapped-delay-line signal processing device.

FIGS. 3a-c generally illustrate various methods for producing variablyreflective optical taps in a continuous length of optical fiber.

FIGS. 4a-b generally illustrate optical fibers prepared with reflectorspositioned at angles relative to the fiber axis.

FIG. 5 schematically illustrates a general embodiment of a transversalfilter which can be implemented in fiber optic form using theconfiguration of FIG. 2.

FIG. 6 illustrates an interferometer incorporated in a continuous lengthof optical fiber.

FIGS. 7a-b schematically illustrate a series of variably reflectiveoptical taps as they may be distributed along a continuous length ofoptical fiber such as to form a progressively monitorable fire alarmsystem.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 generally illustrates one embodiment of the present invention inwhich reflectors may be formed in a continuous length of optical fiber.In one preferred embodiment, reflectors may be formed by the fusionsplicing of two graded index multimode fibers, one of which is coated onthe end with a TiO₂ layer or film. Other preferred embodiments make useof a single mode fiber, and may incorporate a variety of coatingmaterials.

Referring to FIGS. la-1e, an optical fiber 2 is first prepared bycleaving and/or polishing the ends of the fiber 2 with a scribing tool 1or the like such that the ends are flat and smooth. After taking anynecessary precautions to remove organic films or other contaminants,this fiber 2 is placed in a dc planar magnetron system 6 where a TiO₂film 4 is deposited on the ends by sputtering in a 70% argon-30% oxygenproduced atmosphere.

The prepared, coated fiber 2 is then removed from the vacuum system 6where it is placed end-to-end with an uncoated prepared fiber 3 in asplicing unit which uses an electric arc between electrodes 8 and 9.This splicing unit, e.g. a Siecor Model M-67 fusion splicer, is operatedat a much lower arc current and arc duration (typically, 5 mA and 0.3s)than the recommended value of 14 mA and 1.5 s for splicing uncoatedfibers. A number of splicing pulses in sequence are used to produce eachreflector 10.

It is envisioned that many other reflectively dissimilar materials mightbe deposited on the prepared fiber for use in making optical reflectors.Such materials include Zn, ZnO, Ta, Al, A1₂ O₃, Ag, Au, V, VO₂, and V₂O₅. Additionally, these materials may be deposited using alternatemethods of thermal evaporation or electron beam evaporation.

The duration, current, and number of discharges are parameters of theprocess which can be varied to obtain optimum fiber strength and splicereflectivity. If desired, the reflectivity of the splice can bemonitored during the fabrication process using an optical time domainreflectometer (OTDR) as illustrated in FIG. 2. In such a setup, shortpulses at 0.83 μm from a diode laser 12 are injected into an opticalfiber 2, which has been provided with a series of dielectric mirrors 7.The optical pulses reflected from a beam splitter 15, are then monitoredwith a silicon avalanche photodiode 14. It should be understood thatother laser wavelengths, such as 1.3 μm, and other photodetector types,such as germanium avalanche photodiodes, may be used. It should befurther appreciated that a fiber optic directional coupler could be usedas a beam splitter.

FIGS. 3a-3c illustrate two distinct methods for forming variablyreflective taps in a continuous length of optical fiber. As noted, somevanadium oxides undergo a thermally induced dielectric to metaltransition, thereby enabling the construction of variably reflectivemirrors. FIG. 3a illustrates a variable VO₂ or V₂ O₅ reflector 20 formedin a length of optical fiber 2. Placed across this reflector 20 is aresistive film or wire 22 which is formed in contact with the fiber 2.The resistive film or wire is preferably formed of A1, W, Cr, Ti, oralloys thereof. When an electrical current is placed through thisresistive film or wire 22, the fiber 2 is heated sufficiently to drivethe reflector 20 through its phase transition and thus alter its overallreflectivity. When the current is turned off or reduced, the reflector20 cools sufficiently to return through the phase transition to itsoriginal reflectivity.

In an alternate embodiment of the present invention, a variablereflectivity optical tap may be produced by the fusing of a fibersegment between two fiber optic reflectors of nominally equalreflectivity, this fiber segment being disposed between the two mirroredsegments such as to form a Fabry-Perot cavity. These segments may beprepared, coated and fused together according to the methodologypreviously described.

By changing the optical path length of this cavity, it is possible toadjust the reflectivity in a range between a minimum value near zero anda maximum value approximately 4 times the reflectivity of one of themirrors in the absence of such a cavity. This optical path lengthΔL_(opt) may be changed by heating the fiber, where the change inoptical path length ΔL_(opt) is then determined by the relationshipΔL_(opt) =2Δ(nL), where n is the reflective index, and L is the lengthof the fiber. This optical path length may also be changed by applyingan electric field across the fiber in order to change its refractiveindex. This change in path length is dictated by the Kerr effect and maybe expressed by ΔL_(opt) =KE², where E is the applied electric field andK is a constant which depends on the fiber dimensions, and upon the Kerrconstant of the fiber material.

FIGS. 3b and 3c generally illustrate a second method of producingvariable reflective optical taps employing the aforedescribed method. Asseen in FIG. 3b, two reflectors 30 and 32 of nominally equalreflectivity are separated by a segment of fiber 36 in order to form aFabry-Perot cavity. Across this cavity is placed a resistive film orwire 40 which is in contact with the film segment 36. The optical pathlength of this cavity may be charged by heating the fiber 36 such as tochange its refractive index. Alternatively, and as seen in FIG. 3c, anelectron field may be created by applying a voltage between electrodes41 and 42 across the segment 36 to induce a refractive index change.Using this Fabry-Perot cavity, it is possible to adjust the reflectivityof the resultant optical splice in a range between a minimum value nearzero and a maximum value approximately four times the reflectivity ofone of the reflectors in the absence of such a cavity.

The fabrication of reflectors oriented at an angle to the fiber axis maybe seen in FIGS. 4a and 4b. These reflectors are produced by depositingseveral films of a high and low refractive index on the fiber ends priorto splicing. In these figures, a multilayer mirror 60 is created in anoptical fiber 2, said mirror 60 prepared with its reflective surface atan angle 62 to the fiber axis 64.

FIG. 5 schematically illustrates a general embodiment of a transversalfilter which can be implemented in fiber optic form using the generalconfiguration of FIG. 2.

Referring to FIG. 6, Fabry-Perot interferometers 13 consisting of singlemode fibers 3 with mirrors 11 disposed at the ends, have use asinterferometric sensors, and as discriminators for coherentcommunication systems. Using the aforedescribed method, aninterferometer 13 may be incorporated in a continuous length of opticalfiber 3 as illustrated in FIG. 6. In yet other embodiments, low-finesseFabry-Perots, monitored in reflection, may be used as temperaturesensors such as that illustrated in FIG. 7.

FIG. 7 illustrates yet another embodiment of the present invention inwhich variably reflective optical taps are distributed along acontinuous length of optical fiber. It has been established that thefractional charge in the refractive index of SiO₂ fiber is approximately10⁻⁵ /° C. Therefore the reflectance of a Fabry-Perot cavity 10 cm goesthrough one complete cycle over a 6.5° temperature change at l.3 μm. Ifa pulsed light source 80 is coupled to the fiber-tap system and linkedby a fiber optic coupler 81 to a receiver (not shown), a progressivelymonitorable fire alarm system may be formed. In such a system,reflectors 9 undergoing a temperature transition may be identified bytheir higher power signatures as illustrated in FIG. 7b.

Yet another form of such a temperature sensor would make use of a phasetransition material such as VO₂ to form the reflectors 9. A reflectivechange for a particular reflector would occur when it is heated abovethe transition temperature, which in the case of VO₂, is 68° C. .

What is claimed is:
 1. A method of forming variably reflective opticaltaps, comprising:forming two optical mirrors along a continuous lengthof optical fiber such that the mirrors are separated by a length ofoptical fiber, the combination forming a Fabry-Perot cavity; depositingan electrically resistive medium on the fiber between the two mirrors,such that the reflectivity of the cavity will vary upon the passage ofelectrical current through said medium.
 2. The method of claim 1 wherethe resistive medium is a wire or film.
 3. The method of claim 1 wherethe resistive medium is formed of Al, W, Ti, Cr or alloys thereof. 4.The method of claim 1 where the optical mirrors are formed in theoptical fiber by:preparing the ends of two optical fibers by cleaving orpolishing; placing the prepared ends of these fibers in a vacuum system;coating the prepared ends of those fibers with a metallic or dielectricmaterial; fusing the coated prepared ends of these fibers with a secondfiber until the reflectivity of the fused region reaches a desiredvalue.
 5. A method of forming variably reflective optical taps,comprising:depositing a vanadium oxide on one prepared end of a firstoptical fiber; splicing the first fiber to a second fiber so as to forman optical reflector; applying an electrically resistive medium to thefibers in the vicinity of the optical reflector such that thereflectivity of the reflector may be varied upon the passage of anelectric current through said medium.
 6. The method of claim 5 where theresistive medium is a wire or film.
 7. The method of claim 6 where thefilm is formed of Al, W, Ti, Cr or alloys thereof.
 8. The method ofclaim 5 where the optical taps are formed by:preparing one end of afirst optical fiber by cleaving or polishing; placing said first fiberin a vacuum system; coating the prepared end of the first fiber with avanadium oxide; fusing the coated, prepared end of the first fiber withthe end of a second fiber.
 9. The method of claim 5 where the vanadiumoxide is VO₂ or V₂ O₅.
 10. A method of forming variably reflectiveoptical taps, comprising:forming two optical mirrors along a continuouslength of optical fiber such that the mirrors are separated by a lengthof optical fiber, the combination forming a Fabry-Perot cavity;depositing conductive electrodes on opposite sides of the fiber suchthat the reflectivity of the cavity will vary upon the application of apotential difference to the electrodes.